The present invention relates generally to the field of anti-counterfeiting and authentication methods and devices and, more particularly, to methods and security devices for authenticating security documents and valuable products by revealing the shape level lines of a spatial elevation profile.
Counterfeiting of security documents such as bank notes, checks, certification documents, identification cards, passports, travel documents, tickets, etc. has become a serious problem, due to the availability of high-quality and low-priced color photocopiers and desktop publishing systems. The same is also true for valuable products such as CDs, DVDs, software packages, prescription drugs, watches, beverages, foodstuff, cosmetics, clothes, fashion articles etc. that are often counterfeited. The present invention discloses a novel security element and authentication means offering enhanced security for security documents and valuable products which need to be protected against counterfeits.
Various means have been introduced in the prior art for counterfeit prevention. Existing anticounterfeit and authentication means include the use of special paper, special inks, watermarks, micro-letters, security threads, holograms etc. Nevertheless, there is still a need to introduce further security elements, which do not considerably increase the cost of the produced documents or valuable products.
Prior art “phase shift” based methods reveal a latent binary image whose existence, and whose presence is used as a means of authenticating a document. One known method in which a latent binary image is made visible consists in encoding that latent image within a document (see background of U.S. Pat. No. 5,396,559 to McGrew, background of U.S. Pat. No. 5,901,484 to Seder, U.S. Pat. No. 5,999,280 to P. P. Huang, U.S. Pat. No. 6,104,812 to Koltai et. al., and U.S. patent application Ser. No. 09/810,971 Assignee Trustcopy). In “phase shift” based methods, a base layer made of a line grating, or respectively a periodic dot screen is printed on the document, but within the predefined borders of the binary latent image, i.e. on the latent image foreground, the same line grating (respectively, the same dot screen) is printed at a different phase, generally shifted by half a period. Close to the borders of the latent image, the line screen, respectively the dot screen, may be printed at intermediate phases (see U.S. Pat. No. 6,252,971 B1 to Shen-ge Wang). For a layman, the foreground of the latent image printed on the document is difficult to distinguish from its background; but when a revealing layer comprising an identical, but non-shifted line grating or grating of lenticular lenses, respectively a dot screen, is superposed on the document, the latent image pre-designed on the document becomes recognizable, since, within its pre-defined borders, the revealed binary latent image (foreground) appears at a different phase, i.e. at a different intensity compared with the background intensity.
Such phase shift techniques are characterized by the fact that the boundaries of the revealed latent image don't move when displacing the revealing layer on top of the base layer. One limitation of these phase shift techniques resides in the fact that photocopying does generally not destroy the line grating, respectively the dot screen, printed at different phases on the latent image background and foreground. A second limitation resides in the fact that it is relatively easy to recover a binary latent image by revealing it with a revealing line grating of a period close to the line screen, respectively dot screen period. With standard desktop publishing software, counterfeiters may then recreate a similar latent binary image by combining a periodic line grating, respectively dot screen, with the same periodic line grating, respectively dot screen, shifted by half a period, inserted within the borders of the binary latent image.
A variation of the phase shift technique relying on a phase sampling technique is described in U.S. Pat. No. 5,708,717 to Alasia. A further variation of the phase shift technique using conjugate halftone screens is described in U.S. Pat. No. 5,790,703 to Shen-ge Wang. Additional variations of the phase sampling techniques comprising screen element density, form, angle position, size and frequency variations are described in U.S. Pat. No. 6,104,812 to Koltai et. al. A further variation of the phase shift technique consists in having similar line segments printed in registration on two sides of a thick transparent layer: thanks to the parallax effect, the superposition of both layers can be viewed either in phase or out of phase depending on the observation angle, see U.S. Pat. No. 6,494,491 B1 to P. Zeiter et al. A further variation of the phase shift technique consists in printing line segments at different pseudo-random phases in the foreground and the background of a latent image. In the background of the latent image, the identical line segments are printed in registration on the two sides of a security document. In the foreground of the latent image, the identical line segments are printed in registration, but one side of the document is printed at complementary intensities (black instead of transparent and transparent instead of black). In case of misregistration between the line segments printed on both sides of the document, the latent image is not apparent any more (patent application Ser. No. 10/284,551 to Z. Fan et. al.).
The present invention distinguishes itself from prior art phase shift techniques by the fact that it does not embed a hidden latent image within an image and therefore also does not reveal such a latent image. In the present invention, an elevation profile is embedded within one of the layers and the elevation profile's level lines are revealed thanks to the superposition of the two layers.
Prior art “moiré based” methods rely on the superposition of a dot screen (U.S. Pat. Nos. 6,249,588, 5,995,638, and 6,819,775, to Amidror and Hersch), respectively a band grating (U.S. patent application Ser. No. 10/270,546 and U.S. patent application Ser. No. 10/879,218 to Hersch and Chosson) incorporating within the replicated dots, respectively within the replicated bands, variable intensity shapes, and a revealing layer made of a dot screen, respectively a line grating. The revealed moiré shapes are enlarged and transformed instances of the replicated variable intensity shapes. In contrast to these moiré based methods, in the present invention, the shapes of the revealed level lines are not enlarged instances of replicated base layer shapes, but look like offset lines of the shape boundaries from which the elevation profile is derived that is embedded into one of the layers (see section “Detailed description of the invention”).
Chapter 10 of the book by I. Amidror, The Theory of the Moiré Phenomenon, Kluwer, 2000, entitled “Moiré between repetitive non-periodic layers” describes the theory of the superposition of curvilinear line gratings by relying on Fourier series decomposition and spectral domain analysis. Chapter 11 of the same book gives an overview over the indicial method enabling obtaining the geometric layout of the superposition of curved line gratings. In problems 11.4 and 11.5 of Chapter 11 and in the paper by J. S. Marsh, Contour Plots using a Moiré Technique, American Journal of Physics, Vol. 48, Jan. 1980, 39-40, a moiré technique is described for drawing the contour plot of a function g(x,y) which relies on the superposition of a straight line grating and of a curved line grating whose lines have been laterally shifted by an amount equal to g(x,y). These book chapters, together with problems 11.4, 11.5 and the paper by J. S, Marsh however (a) do not consider generating a shape elevation profile from a preferably bilevel motif shape image, (b) do not mention the possibility of having level lines moving between shape borders and the shape centers and (c) do not consider contour plots of a function as a means of authenticating a security document or a valuable product.
The geometric properties of the moiré produced by the superposition of two rectilinear or curvilinear line gratings are described by K. Patorski, The moiré Fringe Technique, Elsevier 1993, pp. 14-16. Moiré fringes (moiré lines) produced by the superposition of two line gratings (i.e. set of lines) are exploited for example for the authentication of bank notes as disclosed in U.S. Pat. No. 6,273,473, Self-verifying security documents, inventors Taylor et al. Neither Patorski's book, nor U.S. Pat. No. 6,273,473 consider modifying a line grating according to a shape elevation profile nor do they consider generating a shape elevation profile from an initial, preferably bilevel, motif shape image. They also don't mention the possibility of having, by superposing base and revealing layers, level lines moving between motif shape boundaries and motif shape centers.